Bone anchoring device

ABSTRACT

A bone anchoring device includes a bone anchoring element having a shaft and a head; and a receiving part for coupling a rod to the bone anchoring element. The receiving part includes a first portion with a U-shaped recess for receiving the rod, and a second portion for accommodating the head, the second portion having a free end and being flexible so as to allow introduction and clamping of the head. The bone anchoring device also includes a locking ring embracing the second portion. The head is locked by means of exerting pressure with the rod onto the locking ring resulting in compression of the second portion of the receiving part and wherein the locking ring is mounted to the receiving part from the free end of the second portion.

RELATED APPLICATION(S)

This application is a continuation of allowed U.S. patent applicationSer. No. 12/183,932, filed Jul. 31, 2008 now U.S. Pat. No. 8,192,470,which claims the benefit of U.S. Provisional Patent Application Ser. No.60/953,170, filed Jul. 31, 2007, the contents of which are herebyincorporated by reference in their entirety, and claims priority fromEuropean Patent Application EP 07 015 041.2, filed Jul. 31, 2007, thecontents of which are hereby incorporated by reference in theirentirety.

BACKGROUND

The present application relates to a bone anchoring device.

Various designs of polyaxial bone screws are known wherein each designhas particular characteristics and wherein different locking principlesare used.

DE 43 05 576 C1 describes a polyaxial bone screw having a screw elementand a receiving part and a pressure element acting onto the head of thescrew element to press it down against the seat in the receiving part inorder to lock the rotational position of the head. The bone screwusually is preassembled so that the screw element is screwed into thebone with the receiving part mounted on the screw element.

Other bone screws are known wherein the head is clamped from the side tolock the rotational position. EP 0 242 708 A2 describes a bone screwwith a receiving part consisting of two spherically-shaped halvespressing from two lateral sides onto the spherical head. The halves areheld together in their lower portion by a ring.

U.S. Pat. No. 5,672,176 discloses a bone screw with a receiving partwith a conically shaped seat and a conically-shaped pressure elementwhich exerts pressure onto the head from above and from the side. If thecone angle has a value lying within a specific range self-locking of thepressure element within the receiving part takes place which allows topreliminary lock the head within the receiving part while the rod isstill moveable in order to allow the adjustment of its position.

EP 0 951 245 B1 describes a bone screw with a receiving part having aconically-shaped cavity accommodating the head wherein a spring chuck isprovided in the cavity which is downwardly and radially compressible toclamp the head. With this bone screw it is possible to click thereceiving part onto the head of the screw element which allows to screwthe screw element into the bone and to connect it to the receiving partthereafter.

U.S. Pat. No. 5,728,098 discloses a bone screw for connection to aspinal rod comprising a screw element and a receiver member which hasslits provided at the bottom of the rod receiving channel and whereintwo locking rings made of a shape memory alloy are provided one at thelower side of the receiver member and one of the upper side. The lockingrings contract about the portions of the receiver member when thetemperature is elevated so that the rod is clamped in the channel.

U.S. Pat. No. 5,549,608 discloses a polyaxial bone screw with a screwelement with a spherical head and a coupling element to couple the screwelement to a spinal rod. The coupling element has a tapered lowerportion including a slotted interior chamber in which the spherical headis initially polyaxially disposed. The coupling element further has arecess for receiving the head. In addition, a locking ring surroundingthe lower portion of the coupling element and a cylindrical rod securingsleeve which fits over the coupling element are provided. A locking nutis used to exert pressure onto the rod securing sleeve. The head islocked in the interior chamber by means of the locking ring which ispressed down by the rod securing sleeve. U.S. Pat. No. 5,575,792discloses a similar device using a hook instead of a bone screw. Incontrast to the afore-mentioned bone screw the rod securing sleeve isomitted and the rod directly presses onto the locking ring. The lockingring has to be placed onto the coupling element from above. It is notsecured against escaping towards the upper end and against rotation whenthe rod is not inserted. Furthermore, the size of this known boneanchoring device is quite large as the locking ring and the locking nutextend substantially outward from the diameter of the coupling element.

Based on the foregoing, there is a need to provide an improved boneanchoring device which comprises a minimum number of parts and which hasa small size while simultaneously providing a safe locking and can beused as a modular system.

SUMMARY

A bone anchoring device according to aspects of the disclosure includesa bone anchoring element and a receiving part for connection to a rod.The head of the bone anchoring element is locked in the receiving partby compression of a portion of the receiving part laterally surroundingthe head by means of a locking ring which is pressed down by the rod.The portion of the receiving part which clamps the head is designed soas to allow the locking ring to be mounted from the free end of theportion. The bone anchoring device can be realized, for example, in formof a polyaxial bone screw allowing a pivotal movement of the head or inform of a monoaxial bone screw allowing a rotational movement of thehead around a single axis in the unlocked state, respectively.

The bone anchoring device according to the aspects of the disclosureincludes only few elements which reduces the costs of manufacturing andfacilitates handling. It makes use of the principle of clamping the headof the bone anchoring element circumferentially from the lateral sideswhich reduces the force necessary to safely clamp the head. The designof the bone anchoring device allows to further reduce the dimension interms of height as well as in terms of the diameter which makes itparticularly suitable for applications where small-sized anchoringdevices are required such as in the field of cervical spine surgery orpediatric applications, trauma and minimal open applications for bonesurgery.

The receiving part can be clicked onto the head of the bone anchoringelement at any time before and during surgery. Therefore, it is forexample possible to first anchor the bone anchoring element in the boneand thereafter connect it to the receiving part and the rod.

By selecting an appropriate angle of the conically-shaped lower portionof the receiving part and the locking ring, it is possible to achieve aself-locking of the head in the lower portion. Alternatively, this canbe achieved by roughening the connecting surfaces of the locking ringand the receiving part. This allows secondary adjustments of the rodwithout loosening the fixation of the head.

Further features and advantages of the invention will become apparentfrom the description of embodiments using the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective exploded view of a first embodiment of thebone anchoring device.

FIG. 2 shows a perspective view of the bone anchoring device of FIG. 1in an assembled state.

FIG. 3 shows a side view of the bone anchoring device of FIG. 2.

FIG. 4 shows a partial sectional view of the bone anchoring device ofFIGS. 1 to 3 in a section perpendicular to the rod axis.

FIG. 5 shows a partial sectional view of the bone anchoring deviceaccording to FIGS. 1 to 3 the section being made in a plane containingthe rod axis.

FIG. 6 shows a perspective view of the receiving part of the boneanchoring device according to the first embodiment.

FIG. 7 shows a side view of the receiving part of FIG. 6.

FIG. 8 shows a side view of the receiving part of FIG. 7 rotated by 90°.

FIG. 9 shows a top view of the receiving part of FIG. 6.

FIG. 10 shows a bottom view of the receiving part of FIG. 6.

FIG. 11 shows a perspective exploded view of the bone anchoring deviceaccording to a second embodiment.

FIG. 12 shows a partial sectional view of the bone anchoring device ofFIG. 11 in an assembled state, the section being perpendicular to therod axis.

FIG. 13 shows a partial sectional view of the bone anchoring device ofFIG. 12, the section being taken in a plane containing the rod axis.

FIG. 14 shows a sectional view of the receiving part of a thirdembodiment of the bone anchoring device.

FIG. 15 shows a sectional view of the bone anchoring device according tothe third embodiment.

FIG. 16 shows a modification of the receiving part with the locking ringof the previous described embodiments.

FIG. 17 shows an enlarged portion 17 of FIG. 16.

FIG. 18 shows a further modification of the bone anchoring device.

FIG. 19 shows a partial sectional view of a further embodiment.

FIG. 20 shows a perspective view from above onto the locking ring of theembodiment according to FIG. 19.

FIG. 21 shows a sectional view of the locking ring of FIG. 20 along lineA-A.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIGS. 1 to 5, the bone anchoring device according to a firstembodiment includes a bone anchoring element 1 in the form of a bonescrew having a threaded shaft 2 and a spherical segment-shaped head 3.The head 3 has a recess 4 for engagement with a screwing-in tool. Thebone anchoring device further comprises a receiving part 5 for receivinga rod 6 to connect it to the bone anchoring element 1. Further, aclosure element 7 in form of an inner screw or set screw is provided forsecuring the rod 6 in the receiving part 5. In addition, the boneanchoring device comprises a locking ring 8 for locking the head in thereceiving part 5.

As can be seen in particular in FIGS. 6 to 10, the receiving part 5comprises a first portion 9 which is substantially cylindrical and whichhas first end 9 a and an opposite second end 9 b. The first portion 9has a coaxial first bore 10 provided at the second end 9 b. The diameterof the first bore 10 is smaller than the diameter of the head 3 of thebone anchoring element. The first portion 9 also comprises a coaxialsecond bore 11 extending from the first end 9 a to a distance from thesecond end 9 b. The diameter of the second bore 11 is larger than thatof the first bore 10 and larger than the diameter of the rod 6. AU-shaped recess 12 is provided in the first portion which extends fromthe first end 9 a to the second end 9 b the diameter of the U-shapedrecess being slightly larger than the diameter of the rod 6 in such away that the rod 6 can be placed in the recess and is guided therein. Bymeans of the U-shaped recess 12 two free legs 12 a, 12 b are formed onwhich an internal thread 13 is provided. The internal thread can be ametric thread, a flat thread, a negative angle thread, a saw-tooththread or any other thread. Preferably, a thread form such as a flatthread or a negative angle thread is used which prevents splaying of thelegs 12 a, 12 b when the inner screw 7 is screwed-in. The depth of theU-shaped recess is such that the rod 6 and the inner screw 7 can beinserted between the legs. Between the bottom of the recess 12 and thelegs 12 a, 12 b a flat section 14 is provide forming the end of the bore11.

The first portion 9 of the receiving part 5 further comprises aplurality of coaxial slits 15 extending from the second end 9 b to adistance from the first end wherein the distance correspondsapproximately to the length of the internal thread 13. The slits 15 areopen at the second end 9 b and extend, as can be seen in particular inFIGS. 6, 9 and 10 through the flat section 14 and the U-shaped recess12. At least one slit 15, and preferably more than one slit is providedon either side of the U-shaped recess. The number of slits is selectedaccording to the degree of elasticity which shall be provided by theslits. The number of slits may depend on the material and the wallthickness and/or other factors.

Adjacent to the second end 9 b the receiving part 5 includes a secondportion 16 providing an accommodation space for the head 3 of the boneanchoring element 1. The second portion 16 has a conically-shaped outersurface including a cone angle α which tapers towards the second end 9 band which has a free end 17 opposite to the second end 9 b. As can beseen in particular in FIG. 4, the outer diameter of the first portion 9at its second end 9 b is larger than the outer diameter of the secondportion 16 adjacent to the second end 9 b and also larger than the outerdiameter of the second portion at its free end 17. Hence, the secondportion 16 is recessed with respect to the first portion 9. The secondportion 16 has an internal hollow spherical section 18 forming a seatfor the spherical segment-shaped head 3 of the bone anchoring element 1.The hollow spherical section 18 is dimensioned in such a way that itencompasses the head 3 of the bone anchoring element from the sidecovering a region including the largest diameter of the head 3. As canbe seen in FIGS. 1, 6, 9 and 10 cuts 24 are provided in the firstportion on either end of the channel formed by the recess 12.

As can be seen in particular in FIGS. 6 to 10, a plurality of slits 19are provided which are open to the free end 17 and extend from the freeend 17 to the second end 9 b of the first portion and continue in theslits 15 of the first portion thereby forming a continuous slitextending from the free end 17 of the second portion into the firstportion. The number of slits 19 may be equal to the number of the slits15, however, it can be smaller or larger depending on the desiredelasticity of the second portion 16. In addition, slits 20 are providedon the side of the second portion 16 which is adjacent to the U-shapedrecess 12 of the first portion. The slits 20 end at a distance from thesecond end 9 b. The elasticity of the second portion 16 is such that thehead 3 of the anchoring element can be inserted by expanding the secondportion and can be clamped by compressing the second portion. The slits15 in the first portion facilitate mounting of the receiving part ontothe head 3 manually, for example at any time before or during surgery.The dimension of the head 3 and the elasticity of the second portion isdesigned such that when the head is introduced into the second portionit is temporarily held in a preliminary angular holding position due tofrictional forces. This preliminary holding position can be selected byrotating the receiving part against the head. It prevents anuncontrolled movement of the receiving part once the bone screw isimplanted and the receiving part is put onto the head.

The locking ring 8 will now be described with reference to FIGS. 1, 4and 5. The locking ring 8 has a substantially cylindrical outer surfacewith an outer diameter corresponding to the outer diameter of the firstportion 9 of the receiving part 5. The height of the locking ring 8 inan axial direction is smaller than that of the second portion 16 of thereceiving part 5 so that, as shown in FIG. 4, there is a distancebetween the locking ring 8 and the second end 9 b of the receiving part5 when the locking ring 8 is in its position clamping the head 3.

The elasticity of the second portion 16 and the size of the secondportion at the free end 17 allows to mount the locking ring by clickingit from the free end 17 onto the second portion 16. Since the outerdiameter of the second portion is smaller than that of the first portion9, the locking ring does not project beyond the first portion in aradial direction.

The locking ring 8 has on its inner side a first section 8 a andadjacent thereto a second section 8 b. The first section 8 a isconically-shaped corresponding to the conical outer wall of the secondportion 16 of the receiving part 5. The first section 8 b iscylindrical. As can be seen in particular in FIG. 4, the dimensions ofthe first section 8 a and the second section 8 b of the locking ring aresuch that the locking ring 8 can slide along the outer surface of thesecond portion 16 and thereby compressing the second portion 16increasingly when sliding downward. If the rod is not yet inserted ornot pressed into the recess 12, the locking ring is movable between afirst position limited by the second end 9 b which acts as a stop andthe free end 17 of the second portion which prevents escaping due to theconical shape.

If the cone angle α is selected to be between approximately 2° and 10°,a self-locking can be achieved between the locking ring and the secondportion which means that an additional force would be required to loosenthe locking ring once it has clamped the head 3. If the cone angle α islarger than approximately 10°, no self-locking effect is present. Ofcourse, the angles at which a self-locking effect can be achieved dependon the materials used and the treatment of the surfaces engaging eachother.

As can be seen in particular in FIGS. 1, 2 and 5, the locking ring 8further comprises on its side facing the second end 9 b two projections21 located diametrically opposite to each other. The projections 21 havesuch a height that they project above the bottom of the U-shaped recess12 and extend into the cuts 24 when the locking ring is in a position inwhich the head 3 is not yet clamped. The free end 22 of the projections21 can be curved with a curvature corresponding to that of the rod 6.The locking ring is arranged in such a way around the second portion 16of the receiving part 5 that the projections 21 are located at thepositions of the U-shaped recess 12. By means of this, the projections21 which project into the U-shaped recess 12 prevent the locking ringfrom rotating when the rod is not inserted.

The inner screw 7 has a thread cooperating with the internal thread 13provided on the legs. If a thread form which prevents the legs fromsplaying is used, a single closure element such as the inner screw 7 issufficient. This reduces the size of the bone anchoring device in aradial direction.

The receiving part, the locking ring, the inner screw and the boneanchoring element are made of a biocompatible material, for example oftitanium or stainless steel or a biocompatible plastic material withsufficient strength.

The bone anchoring device may be preassembled with the locking ringwhich is mounted on the second portion 16 of the receiving part 5 fromthe free end 17. Alternatively, the bone anchoring element 1 can bepreassembled with the receiving part 5 and the locking ring 8.

The bone anchoring device can be used in several ways. In one way of usethe bone anchoring element, the receiving part and the locking ring arepreassembled. The bone anchoring element is screwed into the bone withthe receiving part mounted to the anchoring element. The recess 4 of thehead can be accessed with a screwing-in tool through the first bore 10.The locking ring 8 is in its first position close to the second end 9 bwhere does not clamp the head 3. In this state, the head 3 is pivotablyheld in the second portion 16 which allows the receiving part 5 to bealigned to receive the rod. Once the correct position of the rod withrespect to other bone anchoring devices is achieved, the inner screw 7is screwed between the legs until it presses onto the rod. The rod ispressed against the bottom of the U-shaped recess thereby engaging thefree ends 22 of the projections 21, respectively, and shifting down thelocking ring 8.

When the locking ring 8 is moved toward the free end 17 of the secondportion, it compresses the second portion 16 thereby clamping the head3. Since the force which is exerted by the locking ring acts from thelateral side, the force necessary for safely immobilizing the head issmaller than in the case in which the force acts from above on the topof the head 3. This also allows to downsize the device by allowing thewall thickness of the receiving part to be reduced.

Final tightening of the inner screw locks the rod and the headsimultaneously.

In another way of use, only the receiving part 5 and the locking ring 8are preassembled.

The bone anchoring element 3 is first screwed into the bone and then thereceiving part is clicked onto the head 3 while the locking ring 8 is inits first position close to the second end 9 b and does not compress thesecond portion 16. Alternatively, the bone anchoring element 1 and thereceiving part with preassembled locking ring are assembled by thesurgeon by clicking the receiving part onto the head 3. This allows thesurgeon to select the appropriate bone anchoring element in terms ofdiameter and length of the anchoring section.

In yet another way the head can be locked independently of the rod, if,as described above, the cooperating surfaces of the locking ring and thesecond portion are designed so as to allow a preliminary fixation.

FIGS. 11 to 13 show a second embodiment of the bone anchoring device.Portions and elements which are identical to the first embodiment aredesignated with the same reference numerals as in the description of thefirst embodiment. The description thereof will not be repeated.

The second embodiment differs from the first embodiment only withrespect to the bone anchoring element and the accommodation space in thesecond portion 16 of the receiving part. The bone anchoring element 1′has a threaded shaft 2 and a cylindrical head 30. As can be seen inparticular in FIGS. 12 and 13, the accommodation space of the secondportion 16 of the receiving part 5 has a hollow cylindrical inner space181 the diameter of which is slightly larger than the diameter of thecylindrical head 30 in such a way that the cylindrical head 30 can beinserted and guided in the cylindrical section 181 in the unlockedstate. The end of the cylindrical section forms a stop 182 for the head30. The use of the bone anchoring device according to the secondembodiment is similar to that of the first embodiment. The difference isthat the receiving part 5 cannot pivot relative to the bone anchoringelement 1′ but can only rotate in the unclamped state of the head 30.Such a monoaxial rotatable connection between the receiving part 5 andthe bone anchoring element 1′, allows the receiving part to be alignedwith respect to the rod by only rotating it around the screw axis whichmay be useful in certain anatomical situations.

FIGS. 14 and 15 show a third embodiment of the bone anchoring device.Portions and elements which are identical to the first embodiment aredesignated with the same reference numerals as in the description of theprevious embodiments and the detailed description thereof will not berepeated. The receiving part 5 of the third embodiment comprises aninclined free edge 17′ of the second portion 16. As can be seen inparticular in FIG. 14, the inclined free end 17′ defines a plane whichincludes an angle with the plane defined by the first end 9 a of thefirst portion of the receiving part. The hollow spherical section 18′which accommodates the head 3 is therefore shorter on one side comparedto the opposite side.

As can be seen in FIG. 15 this results in a larger pivot angle to oneside as compared to the opposite side. Hence, a polyaxial screw with anasymmetric pivot angle range is provided. The inclined free end 17′ canbe easily manufactured by cutting.

FIGS. 16 and 17 show a modification of the locking ring. The receivingpart 5 is identical to one of the previously described receiving partsand the description thereof will not be repeated. The locking ring 8′has on either side of each projection 21 a lateral projection 23resulting in a diameter of the projection 21 being larger than thediameter of the cuts 24. When the locking ring 8′ is mounted, theprojections 23 snap into the recess 12 due to the elasticity of thereceiving part 5. The projections 23 form a securing means againstfalling off of the locking ring 8′ when the head 3 is not yet inserted.This may be useful when the surgeon handles the parts before thereceiving part 5 is clicked onto the screw.

FIG. 18 shows a further embodiment which differs from the previousembodiments in that the locking ring 80 has no projections 21. As can beseen in FIG. 18, the height of the locking ring 80 is larger and theU-shaped recess 12 extends deeper into the receiving part such that therod 6 can directly press onto the locking ring 80.

FIG. 19 shows a further embodiment which differs from the embodimentaccording to FIG. 1 in the design of the locking ring 800. The receivingpart 5 is identical to one of the previously described receiving partsand the description thereof will not be repeated. The locking ring 800has a lower part 801 which is designed like the locking ring 8 of, forexample, FIGS. 11 to 13 and includes two projections 21 which are offsetby 180°. Furthermore, the locking ring 800 includes an annular edge 802the inner diameter of which is larger than the outer diameter of thefirst portion 9 of the receiving part 5. The height of the annular edge802 can be as large as the height of the projection 21. However, theannular edge 802 can have a height larger than the projection 21. Thethickness of the annular edge 802 is small, so that the overall diameterof the receiving part 5 is not substantially increased.

As can be seen in FIG. 19, the annular edge 802 has such a height thatwhen the locking ring 800 is in its lower most position clamping thehead of the screw 1 the annular edge 802 closes the gap between the mainportion 801 of the locking ring and the lower side of the first portion9 of the receiving part. Therefore, there is not risk of in-growth oftissue or vessels into this place.

Further modifications of the embodiments described are possible. Forexample, the head of the bone anchoring element can have any othershape, such as, for example, a conical shape and the accommodation spacein the second portion 16 of the receiving part is adapted to this shape.In a further modification the receiving part 5 or at least the secondportion 16 are made of a biocompatible plastic material which provideselasticity to a certain degree. In this case, the slits may be omitted.The projections of the locking ring which engage the rod can haveanother shape. For example, the surface of the free end can be flat orotherwise shaped. Instead of selecting the cone angle to achieveself-locking between the locking ring and the second portion apreliminary locking of the head while the rod is still movable can beachieved by providing surface structures on the contacting surfaces suchas ribs or a roughened surface.

What is claimed is:
 1. A bone anchoring device comprising: a boneanchoring element having a shaft for anchoring to a bone and a head; areceiving part for coupling a rod to the bone anchoring element, whereinthe receiving part comprises: a first portion comprising two legs eachhaving a free end and defining a U-shaped recess for receiving the rod;and a second portion defining an accommodation space for accommodatingthe head, the second portion having a free end and being flexible so asto allow introduction of the head into the accommodation space andclamping of the head when the head is in the accommodation space; and alocking ring configured to be mounted to the receiving part from thefree end of the second portion and to be positioned around the secondportion; wherein when the head is in the accommodation space and thelocking ring is at a resting position relative to the receiving part,the second portion of the receiving part compresses the head of the boneanchoring element with a first force to temporarily hold the boneanchoring element at an adjustable angular position relative to thereceiving part by friction; wherein when the head is in theaccommodation space and the locking ring is at a locking positionrelative to the receiving part different from the resting position, thehead is compressed by the receiving part with a second force greaterthan the first force and the angular position between the bone anchoringelement and the receiving part is locked.
 2. The bone anchoring deviceaccording to claim 1, wherein the diameter of the second portion of thereceiving part adjacent to the first portion of the receiving part issmaller than the diameter of the first portion adjacent to the secondportion.
 3. The bone anchoring device according to claim 1, wherein thelocking ring is movable along the second portion of the receiving partbetween the resting position limited by the first portion and thelocking position limited by the free end of the second portion.
 4. Thebone anchoring device according to claim 1, wherein the outer diameterof the locking ring is smaller than or equal to the outer diameter ofthe first portion.
 5. The bone anchoring device according to claim 1,wherein the second portion of the receiving part has a conical outersurface tapering toward the first portion.
 6. The bone anchoring deviceaccording to claim 5, wherein the locking ring has a section with aconically tapering inner surface corresponding to the conical outersurface of the second portion.
 7. The bone anchoring device according toclaim 6, wherein a cone angle is selected to provide self-lockingbetween the locking ring and the second portion.
 8. The bone anchoringdevice according to claim 7, wherein the cone angle is betweenapproximately 2° and approximately 10°.
 9. The bone anchoring deviceaccording to claim 1, wherein the second portion comprises a pluralityof slits open to the free end of the second portion.
 10. The boneanchoring device according to claim 9, wherein at least one of theplurality of slits extends continuously from the second portion into thefirst portion.
 11. The bone anchoring device according to claim 1,wherein the first portion comprises a plurality of slits extending froma distance from the free ends of the legs towards the second portion.12. The bone anchoring device according to claim 1, wherein the head isspherical segment-shaped and the second portion comprises an innersurface with a corresponding spherical portion to allow a pivotalmovement of the head.
 13. The bone anchoring device according to claim1, wherein the head is cylindrically-shaped and the second portioncomprises an inner surface with a corresponding cylindrical portion torestrict the movement of the head to a rotational movement about asingle axis.
 14. The bone anchoring device according to claim 1, whereina closure element, preferably a set screw, is provided for securing therod in the recess.
 15. The bone anchoring device according to claim 1,wherein a plane going through the free end of the second portion isarranged at an angle to an end defined by the free ends of the legs. 16.The bone anchoring device according to claim 1, wherein the locking ringhas a structure for preventing the locking ring from escaping past thefree end of the second portion when the head is not yet inserted intothe second portion.
 17. The bone anchoring device according to claim 1,wherein at least the second portion comprises an elastic plasticmaterial with no slits.
 18. The bone anchoring device according to claim1, wherein when the bone anchoring device is configured to provide thetemporary hold between the bone anchoring element and the receiving partat the adjustable angular position, uncontrolled movement of thereceiving part relative to the bone anchoring element is prevented. 19.The bone anchoring element according to claim 1, wherein the lockingring and the second portion of the receiving part comprise respectivecontacting surfaces configured to come into contact with one another,and wherein at least one of the contacting surfaces comprises a surfacestructure to hold a position of the locking ring relative to thereceiving part.
 20. The bone anchoring device according to claim 19,wherein the surface structure comprises at least one rib or a roughenedsurface.
 21. A method of attaching a bone anchoring device to a bone orvertebrae, the bone anchoring device comprising a bone anchoring elementhaving a shaft for anchoring to a bone and a head, a receiving part forcoupling a rod to the hone anchoring element, wherein the receiving partcomprises a first portion comprising two legs each having a free end anddefining a U-shaped recess for receiving the rod and a second portiondefining an accommodation space for accommodating the head, the secondportion having a free end and being flexible so as to allow introductionof the head into the accommodation space and clamping of the head whenthe head is in the accommodation space, and a locking ring configured tobe mounted to the receiving part from the free end of the second portionand to be positioned around the second portion, the method comprising:attaching the bone anchoring element to a bone or vertebrae; insertingthe head of the bone anchoring element into the second portion of thereceiving part, wherein when the head is in the accommodation space andthe locking ring is at a resting position relative to the receivingpart, the second portion of the receiving part compresses the head ofthe bone anchoring element with a first force to temporarily hold thebone anchoring element at an adjustable angular position relative to thereceiving part by friction; inserting a rod in the U-shaped recess; andexerting pressure on the locking ring by the rod to cause the lockingring to move from the resting position to a locking position, where thereceiving part compresses the head with a second force greater than thefirst force to lock the angular position between the bone anchoringelement and the receiving part.
 22. The method according to claim 21,wherein the head of the bone anchoring element is inserted into thesecond portion of the receiving part prior to the attaching of the boneanchoring element to the bone or vertebrae.